High resolution mass spectrometry is widely used in the detection and identification of molecular structures and the study of chemical and physical processes. A variety of different techniques are known for the generation of a mass spectrum using various trapping and detection methods.
One such technique is Fourier Transform Ion Cyclotron Resonance (FT-ICR). FT-ICR uses the principle of a cyclotron, wherein a high frequency voltage excites ions to move in a spiral within an ICR cell. The ions in the cell orbit as coherent bunches along the same radial paths but at different frequencies. The frequency of the circular motion (the cylcotron frequency) is proportional to the ion mass. A set of detector electrodes are provided and an image current is induced in these by the coherent orbiting ions. The amplitude and frequency of the detected signal are indicative of the quantity and mass of the ions. A mass spectrum is obtainable by carrying out a Fourier Transform of the ‘transient’, i.e. the signal produced at the detector's electrodes.
An attraction of FT-ICR is its ultrahigh resolution (up to 1,000,000 in certain circumstances and typically well in excess of 100,000). However, to achieve such high resolution, it is important that various system parameters be optimised. For example, it is well known that the performance of an FT-ICR cell seriously degrades if the pressure therein rises above about 2×10−9 mbar. This places restrictions on the cell design and upon the magnet that supplies the field to cause the cyclotron motion of the ions. Problems with space charge within the cell (which affects resolution) also affect cell design parameters. Furthermore, when the cell is supplied with ions from an external source, using either electostatic injection to the cell, or using a multipole injection arrangement (see U.S. Pat. No. 4,535,235), it is known that minimization of time of flight effects is desirable.